Farm Machinery
Selection
P
utting together an ideal machinery system
is not easy. Equipment that works best one
year may not work well the next because of
changes in weather conditions or crop production
practices. Improvements in design may make older
equipment obsolete. And the number of acres being farmed or the amount of labor available may
change.
Because many of these variables are unpredictable,
the goal of the good machinery manager should be
to have a system that is flexible enough to adapt
to a broad range of weather and crop conditions
while minimizing long-run costs and production
risks. To meet these goals several fundamental
questions must be answered.
Machine Performance
First, each piece of machinery must perform reliably under a variety of field conditions or it is a
poor investment regardless of its cost.
Tillage implements should prepare a satisfactory
seedbed while conserving moisture, destroying
early weed growth and minimizing erosion potential. Planters and seeders should provide consistent
seed placement and population as well as properly
apply pesticides and fertilizers. Harvesting equipment must harvest clean, undamaged grain while
minimizing field losses.
The performance of a machine often depends
on the skill of the operator, or on weather and
soil conditions. Nevertheless, differences among
machines can be evaluated through field trials,
research reports and personal experience.
Machinery Costs
Once a particular type of tillage, planting, weed
control, or harvesting machine has been selected,
the question of how to minimize machinery costs
must be answered. Machinery that is too large for
a particular farming situation will cause machinery
ownership costs to be unnecessarily high over the
Ag Decision Maker
File A3-28
long run; machinery that is too small may result in
lower crop yields or reduced quality.
Ownership Costs
Machinery ownership costs include charges for depreciation, interest on investment, property taxes,
insurance and machinery housing. These costs
increase in direct proportion to machinery investment and size.
Operating Costs
Operating costs include fuel, lubricants and repairs.
Operating costs per acre change very little as machinery size is increased or decreased. Using larger
machinery consumes more fuel and lubricants per
hour, but this is essentially offset by the fact that
more acres are covered per hour. Much the same is
true of repair costs. Thus, operating costs are of minor importance when deciding what size machinery
is best suited to a certain farming operation.
A detailed procedure for estimating machinery
ownership and operating costs can be found in
AgDM Information File A3-29, Estimating Farm
Machinery Costs (PM 710).
Labor Cost
As machinery capacity increases, the number of
hours required to complete field operations over
a given area naturally declines. Estimates of time
requirements for completing machinery operations
are found in AgDM Information File A3-24,
Estimating the Field Capacity of Farm Machines
(PM 696).
If hourly or part-time hired labor operates machinery, it is appropriate to use the wage rate paid,
plus the cost of any other benefits which may be
provided, as the labor cost. If the farmer-owner or
a hired worker who is paid a fixed wage operates
machinery, then it is proper to value labor at its
opportunity cost, or the estimated return it could
earn if it were used elsewhere in the farm business,
such as in livestock enterprises.
PM 952 Revised April 2015
Page 2
Farm Machinery Selection
Timeliness Costs
Number of Crop Acres
In many cases, crop yields and quality are affected
by the dates of planting and harvesting. This
represents a “hidden” cost associated with farm
machinery, but an important one nevertheless. The
value of these yield losses is commonly referred to
as “timeliness costs.”
As more crop acres are farmed, larger-scale machinery is needed to ensure that planting and
harvesting can be completed in a timely fashion.
An alternative is to acquire a second unit of some
machines, if an additional tractor and operator are
available.
Total Machinery Costs
Labor Supply
Figure 1 illustrates the effect that changes in machinery size have on each type of cost in a typical
situation. For very small machinery (relative to
crop acres), a slight increase in machinery size
can lower timeliness and labor costs significantly,
enough to more than offset the higher fixed costs.
However, as machinery size continues to increase,
the timeliness cost savings diminish, and eventually total costs begin to rise. One objective of
machinery selection, then, is to select machinery
in the size range where total machinery costs are
lowest.
The number of acres that can be completed each
day is the most critical measure of machinery capacity; more than machine width or acres completed per hour. Increasing the labor supply by hiring
extra operators or by working longer hours during
critical periods may be a relatively inexpensive way
of stretching machinery capacity. In addition, the
cost of additional labor only needs to be incurred
in those years in which it is actually used, while
the cost of investing in larger machinery becomes
“locked in” as soon as the investment is made.
On the other hand, extra labor may not always be
available when needed, and working long hours
over several days can present a safety hazard.
Figure 1. Effect of increasing machinery
size on machinery costs.
Machinery
Cost per Acre
$90
$80
Total costs
$70
$60
$50
Minimum cost
point
$40
$30
$20
Labor costs
$10
$0
Tillage Practices
The number of field days needed before planting is
completed depends partly on the number of separate operations completed on each acre. Reducing the number of tillage practices performed or
performing more than one practice in the same trip
effectively decreases the amount of machinery capacity needed to complete field operations on time.
Of course, machinery cost savings from reduced
tillage must be compared to possible increased
chemical costs and effects on yields.
Timeliness costs
Machinery Size
Factors That Affect the Size of
Machinery Needed
Machinery recommendations must be based on the
characteristics of each individual farm. The following factors influence machinery selection, and are
discussed in order of importance.
Crop Mix
Diversification of crops tends to spread out the
periods when timely completion of field operations
is critical. For example, yield reductions due to
late planting begin later for soybeans than for corn.
Harvesting can also be completed over a longer
time period. Thus, growing more than one or two
crops reduces the machinery capacity needed for a
given number of crop acres. However, it may also
require purchasing additional types of machinery,
especially for harvesting.
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Farm Machinery Selection
Weather
Weather patterns determine the number of days
suitable for fieldwork in a given time period each
year. Although actual weather conditions cannot
be predicted far enough in advance to be used as
an aid to machinery selection, past weather records
can be used as a guide. AgDM Information File A325, Fieldwork Days in Iowa (PM 1874) lists the
number of suitable field days expected for different
periods of the year in each of the nine crop reporting districts in Iowa. As a rule of thumb, weather
is suitable for field work about 60 percent of the
time in the spring and about 75 percent of the time
in the fall. This does not take into account time off
for holidays, Sundays or other occasions. Machinery selection should be based on long-run weather
patterns even though it results in excess machinery
capacity in some years and insufficient capacity in
other years.
Risk Management
Fluctuations in the number and occurrence of
suitable field days from year to year cause timeliness costs to vary even when the machinery set,
number of crop acres and labor supply do not
change. Investing in larger machinery can reduce
the variability of net machinery costs by ensuring
that crops are planted and harvested on time even
in years in which there are few good working days.
Machinery fixed costs would be higher with larger
machinery, but they would not fluctuate as long as
the machinery set did not change. Farmers with
high fixed cash flow needs, such as land mortgage
payments, may be willing to pay more (in higher
fixed machinery costs) than other operators for the
“insurance” of not suffering substantial yield losses
due to late planting and harvesting in certain years.
Planting and Harvesting Dates
Long-term studies indicate that corn yields typically start to decline significantly when planting
occurs after May 10 to 14, as shown in Figure 2.
The exact dates will vary from year to year. About
50 percent of Iowa’s corn is normally planted by
this time. One reason for the decline in yield for
late-planted corn is that fewer “heat units” are
available during the growing season, and this influences the rate of crop development.
Figure 2. Estimated corn yield as percent of
maximum, by planting date.
100%
95%
90%
85%
80%
75%
70%
65%
60%
55%
50%
1-May
11-May
21-May
31-May
10-Jun
20-Jun
Source: Iowa State University, Corn Planting Guide, PM 1885
How early to start planting requires considerable judgment. Ideal conditions would be a soil
temperature of 50°F (10°C) or above at planting
depth and a favorable five-day weather forecast. In
most of Iowa, if soil conditions and temperatures
are favorable, starting to plant during the last ten
days of April should be advantageous. In May, the
major consideration should be the condition of the
seedbed.
There is some risk with early planting. Replanting
may occasionally be required, but the long-term
benefits far outweigh this cost. An added benefit
from early-planted corn is lower grain moisture
levels at harvest and reduced drying costs.
Most of the same things can be said about planting soybean varieties. The ideal time for planting
adapted soybean varieties is between May 1 and
May 15. Yields can be expected to decline in most
years if planting occurs after May 20, as Figure 3
shows.
Timeliness losses at harvest are due primarily to
more dropped ears, field shattering and cracked
beans. These losses must be balanced against the
cost of artificially drying grain harvested at a mois-
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Farm Machinery Selection
ture level higher than that required for safe
storage. Some harvesting losses occur because
combining speed is too high or the machine is
poorly adjusted.
Figure 3. Estimated soybean yield as
percent of maximum, by planting date.
100%
95%
90%
Where less than one full-time person is available to
operate machinery, a goal of 25 to 30 days for completing planting and tillage will most often minimize costs. On the other hand, farms with two or
three full-time machinery operators available could
aim to complete this work in less than 20 days.
The machinery sets which minimized total machinery costs were most often able to complete
harvesting of corn and soybeans in 25 to 30 field
days. As with spring work, operators for whom
risk reduction is important should use the lower
end of this range as a goal, although yield losses
from late harvesting are generally not as severe as
from late planting.
85%
80%
75%
70%
65%
60%
55%
50%
1-May
capacity to reduce risk, as well as maintain total
costs at a low level, is to be able to complete tillage
and planting in about 20 field days.
11-May
21-May
31-May
10-Jun
20-Jun
Source: Iowa State University, Soybean Replant Decisions,
PM 1851
How Large Should Machinery Be?
One way to measure the capacity of a set of machinery is by the number of work days required
to complete field operations. This depends on
the number of crop acres, machinery operations
performed, size of the machinery in use, and availability of labor.
In research conducted at ISU, total machinery
costs for Iowa grain farms, including the value of
timeliness losses, were estimated for a number of
different machinery combinations. The effects of
variations in the number of crop acres and labor
supply, were compared.
Under each set of circumstances the machinery set
for which total costs were lowest, including yield
losses due to poor timeliness, was identified. In
some cases, several machinery sets gave nearly identical minimum costs. In approximately 80 percent of
the cases tested, the least-cost machinery sets were
able to complete all tillage and planting operations
in about 20 to 25 field days. A good rule of thumb
for farmers who wish to have sufficient machinery
A number of different machinery combinations
may allow fieldwork to be completed in the same
number of days. In putting together a machinery
set, it is also important to correctly match machinery sizes and tractor power. Using tractors
with horsepower in excess of that required for the
implement being pulled results in excessive depreciation and interest costs, while using too little
horsepower may cause faster engine wearout.
Some farms may not have enough crop acres to
justify owning a full line of machinery, particularly
for harvesting. Custom hiring or leasing certain
machinery operations may lower total costs as
well as provide more flexibility in the amount of
machinery capacity. For a detailed discussion of
custom hiring, leasing, and renting farm machinery see AgDM Information Files A3-33 Combine
Ownership or Custom Hire (PM 786) and A3-21
Acquiring Farm Machinery Services (PM 787).
Field Capacity
To project the number of field days required, it
is necessary to know the field capacity for each
implement. Field capacity usually is measured in
acres accomplished per hour, and is affected by
three variables: width, speed and field efficiency.
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Farm Machinery Selection
Width refers to the effective working width of the
implement, excluding overlapping, and is measured in feet. Speed, which is measured in miles per
hour, refers to a safe operating speed under normal
working conditions. This does not take into account
slowing down to turn at the end of a field.
Field efficiency is the actual field capacity that can
be achieved as a percentage of the maximum theoretical capacity without overlapping, slowing for
turning or stopping to adjust machinery, fill containers, empty hoppers, and make minor repairs.
The formula for estimating field capacity (in acres
per hour) is:
width (ft) x speed (mph)
x field efficiency (%)
8.25 1
= field capacity (A/hr)
For example, assume a 24-foot tandem disk can be
pulled at 6 miles per hour with a field efficiency of
80 percent. Its estimated field capacity is:
Table 1. Default Values For Speed, Field Efficiency and Draft Requirements.
Speed
(mph)
Draft (lb. per
unit of width)
5.0
4.5
4.5
4.0
320
500
800
1200
220 - 430 per foot
350 - 650 per foot
580 - 1,140 per foot
1,000 - 1,400 per foot
5.0
500
200 - 800 per shank
5.5
5.5
5.0
5.0
5.0
6.0
5.0
75
200
325
300
200
50
100
50 - 100 per foot
100 - 300 per foot
250 - 400 per foot
200 - 400 per foot
70 - 300 per foot
20 - 60 per foot
20 - 150 per foot
5.0
4.5
7.5
250
80
84
60 - 300 per foot
40 - 120 per foot
30 - 100 per foot
4.5
4.5
4.5
1500
2000
2600
1,100 - 1,800 per tooth
1,600 - 2,600 per tooth
2,000 - 3,000 per tooth
Planting
Planter only
Planter with attachments
Grain drill
No-till drill
5.0
5.0
5.0
5.0
150
350
70
200
100 - 180 per row
250 - 400 per row
30 - 100 per foot
160 - 240 per foot
Applying Chemicals
Anhydrous ammonia application
4.5
425
375 - 450 per shank
Equipment Name
Tillage
Moldboard plow (16 in. bottom, 7 in. deep)
Light soil
Medium soil
Heavy soil
Clay soil
Chisel-plow
(7-9 in. deep)
Disk
Single gang
Tandem
Heavy or offset
Field cultivator
Spring-tooth harrow
Spike-tooth harrow
Roller or packer
Cultivator
Field (3-5 in. deep)
Row crop
Rotary hoe
Subsoiler (16 in. deep)
Light soil
Medium soil
Heavy soil
Average Range
Source: Hunt, Donnell. Farm Power and Machinery Management, 9th edition, ISU Press, 1995, and Bowers,
Wendell, Machinery Management, Deere and Company, 1987.
The factor 8.25 is a conversion factor calculated by 43,560 sq. feet per acre / 5,280 feet per mile.
1
Page 6
24 feet x 6 mph x 80%
8.25
Farm Machinery Selection
= 14 acres per hour
Suggested values for estimating field capacities
can be found in Table 1, and in AgDM Information
File A3-24, Estimating Field Capacity of Farm
Machines (PM 696).
To estimate the field capacity needed to complete
a certain field operation in a set number of field
days, use the following formula:
acres to cover
(days available x hours
of field time per day)
= field capacity needed
needed to pull a certain implement depends on the
width of the implement, the ground speed, draft
requirement, and soil condition.
The formula for estimating the required horsepower measured at the power take-off (PTO) is:
width (feet) x speed (mph) x
draft (lb./ft.) x soil factor
375
= PTO hp
The soil factors are as follows:
firm soil
tilled soil
sandy or soft soil
2WD
1.64
1.75
2.13
4WOA
1.54
1.61
1.82
4WD
1.52
1.56
1.67
= 3.75 acres per hour
Table 1 shows estimated draft requirements for
various implements. For example, a 24-foot tandem disk being pulled at 6.0 mph by a 4-wheel
assist tractor in firm soil, with a draft requirement
of 200 pounds per foot of width, would need the
following PTO horsepower:
The minimum implement width then can be found
by inverting the formula for field capacity:
(24 ft. x 6.0 mph x 200 lb./ft. x 1.54) = 118 PTO hp
375
For example, to combine 900 acres of corn in 20
field days, harvesting 12 hours per day, would
require a field capacity of:
900 acres
(20 days x 12 hours/day)
(8.25 x field capacity)
(speed x field efficiency)
= width
For the example, if the combine operates at 3 miles
per hour with 70 percent field efficiency, the minimum width is:
(8.25 x 3.75 acres/hour)
= 14.7 feet
(3.0 miles per hour x 70%)
The minimum width is about the size of a six-row,
30-inch corn head.
AgDM Decision Tool A3-24, Estimating the Field
Capacity of Farm Machines, can be used to estimate the acreage capacity of a machinery implement.
Matching Tractor Power and Implement
Size
For tillage and planting implements the size of
the machine that can be used is often limited by
the size of the available tractor. The horsepower
For some implements the width and the draft
requirements are measured in terms of the number
of teeth, shanks or rows rather than feet.
AgDM Decision Tool A3-28, Matching Tractor
Power and Implement Size, can be used to match
the size of a tractor to the size of an implement.
Estimating the Number of Field Days
Required
The following worksheet can be used to estimate
the number of field days required for tillage,
planting and harvesting for a particular farming
operation.
Column 1. List all the field operations to be done
before planting. Include fall and spring tillage,
application of chemicals, and sowing of small
grain or forages. Do not include custom hired
operations.
Column 2. List the total acres to be covered by each
operation. Remember, if some acres have the same
Page 7
Farm Machinery Selection
operation performed on them more than once,
multiply the number of acres by the times over.
Column 3. List the sizes of the machines used for
all operations.
Column 4. List the field capacity of each machine
in acres per hour. Suggestions can be found in
Table 1 or AgDM Information File A3-24, Estimating Field Capacity of Farm Machines (PM 696),
or by using the following formula:
width (ft) x speed (mph) x
field efficiency (%)
= field capacity (A/hr)
8.25
It may be more convenient to skip directly to column 6 and enter the number of acres covered per
day, if this is known.
Column 5. Enter the number of labor hours available per day in the field to perform each tillage and
preplant operation. Do not count time spent on
repairs, transportation of machinery, livestock activities, etc. For planting and harvesting, enter the
number of hours per day the planter or combine
can be used.
Column 6. Multiply column 4 by column 5 to
estimate the number of acres covered per day for
each operation. Decide if this is a reasonable figure
based on experience.
Column 7. Estimate the number of field days
needed for each operation by dividing column 2
by column 6. Then find the total for each group of
field operations.
Use extra lines to estimate how the number of field
days required can be adjusted. Adjustments can be
made by changing machinery size, number of field
operations, number of acres covered, proportion of
acres in each crop, hours available for fieldwork,
or by custom hiring some operations.
This worksheet can also be used to estimate the
number of field days required for harvesting forages. This can be compared to the expected number
of field days available in the appropriate period, as
shown in AgDM Information File A3-25, Fieldwork
Days in Iowa (PM 1874).
AgDM Decision Tool A3-28, Estimating Field Capacity of Farm Machinery, can be used to estimate
the number of field days required to complete field
operations.
Other publications that will help you make good machinery management decisions are:
PM 696 Estimating Field Capacity of Farm Machines
PM 709 Fuel Required for Field Operations
PM 786 Combine Ownership or Custom Hire
PM 787 Acquiring Farm Machinery Services
PM 952 Farm Machinery Selection
PM 1373 Joint Machinery Ownership
PM 1450 Transferring Ownership of Farm Machinery
PM 1860 Replacement Strategies for Farm Machinery
PM 1874 Fieldwork Days in Iowa
Page 8
Farm Machinery Selection
Field Days Worksheet Example—800 acres of corn, 800 acres of soybeans.
(1)
(2)
(3)
(4)
Type of Operation
Total Acres to
be covered by
Implement
Your
Implement
Size
Field Capacity
Acres Per
Hour
(5)
Labor
Available for
Fieldwork
Hours/Day
(6)
Acres
Covered
Per Day
(Col. 4 x Col. 5)
(7)
Field Days
Needed
(Col. 2/Col. 6)
All Tillage and Pre-plant Chemical Application
Apply nitrogen
800
20 ft.
8
12
96
8.3
Chisel plow
800
25 ft.
14
16
224
3.6
Spray
1,600
60 ft.
60
12
720
2.2
Field cultivate
1,600
34 ft.
24
12
288
5.6
Planting
Plant corn
800
16-30
18
16
288
2.8
Plant soybeans
800
16-30
18
16
288
2.8
1,600
60 ft.
60
12
720
2.2
Spray
Total
27.5
Harvesting
Harvest soybeans
800
25 ft.
7
10
70
11.4
Harvest corn
800
8-30
6
10
60
13.3
Total
24.7
Page 9
Farm Machinery Selection
Field Days Worksheet
(1)
(2)
(3)
(4)
Type of Operation
Total Acres to
be covered by
Implement
Your
Implement
Size
Field Capacity
Acres Per
Hour
(5)
Labor
Available for
Fieldwork
Hours/Day
(6)
Acres
Covered
Per Day
(Col. 4 x Col. 5)
(7)
Field Days
Needed
(Col. 2/Col. 6)
All Tillage and Pre-plant Chemical Application
Planting
Plant corn
Plant soybeans
Total
Harvesting
Harvest soybeans
Harvest corn
Total
. . . and justice for all
The U.S. Department of Agriculture (USDA) prohibits discrimination in all its programs and
activities on the basis of race, color, national origin, gender, religion, age, disability, political
beliefs, sexual orientation, and marital or family status. (Not all prohibited bases apply to all
programs.) Many materials can be made available in alternative formats for ADA clients. To
file a complaint of discrimination, write USDA, Office of Civil Rights, Room 326-W, Whitten
Building, 14th and Independence Avenue, SW, Washington, DC 20250-9410 or call 202-7205964.
Issued in furtherance of Cooperative Extension work, Acts of May 8 and July 30, 1914, in
cooperation with the U.S. Department of Agriculture. Cathann A. Kress, director, Cooperative
Extension Service, Iowa State University of Science and Technology, Ames, Iowa.
Prepared by William Edwards
retired extension economist
www.extension.iastate.edu/agdm
store.extension.iastate.edu